Transforming a voltage from a direct current (DC) power supply is typically accomplished by converting the direct current voltage to an alternating current (AC) voltage and then transforming it to the desired voltage. Means for converting the DC voltage to an AC voltage are well-known in the art, and typically comprise an inverter which functions to switch the voltage across a resonant tank circuit in order to create an AC sinusoidal voltage waveform, this voltage waveform then being applied across a transformer. The magnitude of the transformed voltage is controlled by the switching frequency of the inverter. Prior art inverters suffer from inherent disadvantages, including power loss due to switching transients and dynamic instability.
Recent proposals for inverters useful with solar powered ion propulsion systems are based upon an "intelligent" controller circuit in which the switching of the inverter is controlled in response to conditions existing during the previous switching cycle. The "intelligent" controller of the prior art is inherently unstable because its control is based upon the conditions existing in the previous switching cycle. Furthermore, it lacks the ability to switch the inverter output polarity precisely during periods of minimum current, causing significant switching power losses in the inverter.
One example of an "intelligent" controller is disclosed in Schwarz et al, "A 95% Efficient 1 Kilowatt DC Converter", Power Electronics Specialists Conference Record, 1977, pages 125-134. Such "intelligent" controllers are of a necessity complex, another disadvantage.
In some cases, prior art inverters are switched by silicon controlled rectifiers which require the application of a reverse current pulse every half cycle for a minimum period of time, further decreasing the power efficiency and limiting the frequency at which the inverter may be operated.
A purpose of the present invention is to increase the power efficiency of the inverter while at the same time improving its stability and simplifying the control circuitry associated therewith.